The whole name for it is NPC-RC which stands for Negative Peak Conversion-Reduced Carrier. What it does is (as I under stand it) is compresses the negative peaks of your modulation. I had gotten into radios back in the mid 70's and learned a lot from a local tech, then learned more on my own later by experimentation. I even ran a small radio shop out of my house, but a ugly divorce ended that.
Any way I am now getting back into them and unfortunately have never done one of these conversions, but from what I have heard around the net, a lot of people like it. It makes your radio modulate louder without the distortion